The employment of valves which are spring biased to remain closed and are opened by the action of a rotating eccentric shaft or cam acting against the end of the valve stem is well known and widely utilized in the internal combustion engine art but inherently possesses certain disadvantages.
The action of the rotating cam striking the valve stem includes a force component which is transverse to the longitudinal axis of the valve. This force component deleterously effects friction and wear in the valve guide and, after a period of time, substantially reduces efficient valve operation and ultimately may result in the failure of the valve to operate. Also, the non-axial action of the valve induces uneven wear between the valve head and the valve seat and eventually precipitates the destruction of the ability of the valve head to seal against the valve seat. As valve operation continues in the presence of such uneven wear hot spots form between the valve head and valve seat which will quickly effect the metallergic integrity of the valve and expedite failure.
An additional disadvantage of a spring loaded valve is the waste of the energy imparted to the spring by the rotating cam. The energy stored in the spring when it is compressed is not substantially returned to the cam but is dissipated as heat or vibration when the valve seats.
Another difficulty inherent in the spring loaded valve is the internal characteristics of the spring and valve which result in the valve being only partially open for a significant portion of its operational cycle and fully extended for a relatively small portion of the cycle. This characteristic produces relative restriction of gas flow, increased back pressure on the exhaust stroke of the engine and an overall decrease in engine efficiency. In order to obtain greater power output, for applications such as racing engines, it is desirable that an internal combustion engine have a greater rotational velocity range. Operation of an internal combustion engine at increasingly higher rotational velocities will eventually result in the attainment of an engine speed at which the valve spring can no longer respond fast enough to follow the cam. The nonresponsive movement of the spring and valve will decrease the engine's power and efficiency and effectively establish the maximum speed at which the engine can operate.
As the engine's operational maximum revolution per minute (RPM) range is increased the inertial characteristics of the spring and valve make it necessary to select a camshaft that will provide greater valve penetration and one that will open the valve earlier in the operational cycle. Opening the valve earlier in the cycle and increasing the valve penetration consequently results in the valve closing later in the engine's operational cycle. Increasing the open to close time periods of the inlet and exhaust valves of a high RPM engine, which completes a cycle in a shorter period of time than a low RPM engine, inherently requires that the inlet valve begin to open before the exhaust valve closes, resulting in substantial overlapping of the periods where each valves are open. A camshaft which alters low and medium speed valve timing and thus improves the high RPM characteristics will give a lower volumetric efficiency, due to the greater valve overlap, and therefore produce a poorer power output at low and medium speeds than a standard camshaft. Thus it is necessary to compromise between maximum obtainable power and flexibility of performance in lower and medium engine-speed ranges.
To achieve increased valve penetration with the more rapid operational characteristics of high RPM engines it is necessary to increase the strength of the spring so that engagement between the valve and camshaft is maintained through the entire operational cycle. Increasing the spring strength increases the force and energy which must be applied by the cam and correspondingly increases the transverse force component induced by the cam which results in more rapid wear on the valve and valve guide. On the other hand, decreasing the valve penetration restricts the flow of gases and reduces the power of the engine.
A further disadvantage of cam driven spring loaded valves is the inability to adjust valve timing or penetration without removing and exchanging the cam shaft or valve springs.